1. Field of the Invention
The present invention relates to an apparatus for correcting an input offset voltage, and more particularly to an input offset voltage corrector used for adjusting the input offset voltage of an operational amplifier.
2. Description of Related Art
As an input offset voltage is generally used for setting a static DC output voltage to zero or to a certain predetermined value, it is necessary to add the input offset voltage to the DC voltage at an input terminal. If the input stage of an operational amplifier is perfectly symmetrical and the transistor is fully compliant, then the input offset voltage will be equal to zero, but the geometric structure and doping of the chip may have discrepancies due to the change of fabrication process. Thus all operational amplifiers need a small voltage to be added between an inverting input terminal and a non-inverting input terminal to compensate for the incompliance of the chips.
In general, the input offset voltage is one of the important factors we should take into consideration for applying the characteristics of a comparator or an operational amplifier. The magnitude of the input offset voltage will affect the operating point of DC amplification and the maximum amplification factor for the application, or an error may occur after the signal is amplified. Referring to FIG. 1 showing a circuit diagram of a non-inverting amplify circuit, an operational amplifier 10 is used as an application circuit of an amplifier. If a first resistor R1 of the operational amplifier 10 is a 1 kΩ resistor and a second resistor R2 is a 99 kΩ resistor, then the amplification factor of the non-inverting amplify circuit will be equal to 100. The output Vo of the operational amplifier 10 is equal to the sum of the input voltage Vin and the input offset voltage Vio multiplied by 100 (Vo=(Vin+Vio)×100). From this formula, we know that the input voltage Vin is amplified 100 times, and the input offset voltage Vio is also amplified 100 times. Therefore, it is necessary to consider the magnitude of the input offset voltage Vio for the circuit design and try to minimize the input offset voltage Vio, so as to avoid an excessively large error of the output Vo of the operational amplifier 10 caused by an excessively high input offset voltage Vio or to avoid saturating the operational amplifier 10.
Referring to FIG. 2 showing a circuit diagram of a current detection circuit, a comparator is used as an application circuit of an operational amplifier 10. If the current I detected by the circuit falls in the range of 0 A˜20 A, an analog-to-digital converter 21 will be used for reading a current signal into a micro control unit 31. Since the maximum detection current I can be up to 20 A, a 10 mΩ/5 W resistor is used as the current detection resistor R3 to lower power consumption. Therefore, the magnitude of the voltage at the current detection resistor R3 is equal to 0.2V; the voltage at the current detection resistor R3 is amplified 25 times after going through a 1 kΩ fourth resistor R4 and a 24 kΩ fifth resistor R5; and the voltage at an output terminal of the operational amplifier 10 is equal to 0˜5V. However, if the input offset voltage Vio of the operational amplifier 10 is equal to ±30 mV, the error of the current of the micro control unit 31 read by the analog-to-digital converter 21 will be up to ±3 A (30 mV/10 mΩ), and the error percentage is equal to 15%. Therefore, the magnitude of the input offset voltage Vio of the operational amplifier 10 will affect the accuracy of the current read by the micro control unit 31.
In a general semiconductor fabrication process for the micro control unit, the input offset voltage of the operational amplifier is approximately equal to ±30 mV. If it is necessary to design a low input offset comparator or amplifier, a trimming method or a chopper method is generally used for the design. However, if the input offset voltage is corrected by the trimming method, the correction result only can guarantee the performance under the correct conditions, since the input offset voltage will shift with different operating voltages, operating temperatures and input voltages. If the input offset voltage is corrected by the trimming method and the operating conditions are changed, the characteristic of the operational amplifier will deteriorate and a larger error will result. If the chopper method is adopted, the circuit requires additional clock circuit and filter, thus causing a higher cost for the circuit. Furthermore, the operating bandwidth and circuit response speed cannot be enhanced to meet the requirements of the application of a high-speed comparator or amplifier.